1. Field of the Invention
The present invention relates to signal repeaters and, more particularly, to expendable signal repeaters for use in unattended radar applications.
2. Description of the Prior Art
In the prior art, it has been recognized that it would be useful to deploy signal repeaters over a given terrain for the purpose of confusing radar sets interrogating that terrain, causing them to detect targets which, in fact, did not exist. Typically, these repeaters would receive the signals transmitted by the interrogating radar set, delay them for a time, and transmit them back to the radar set at a higher amplitude causing the radar to detect a relatively large target at a more distant range than the true range of the signal repeater. To take advantage of the utility of amplifying these signals at lower frequencies than the radio frequencies of signals transmitted by the radar, the prior art signal repeaters typically converted the radio frequency of received signals to an intermediate frequency by mixing the signals with the fundamental frequency of a local oscillator. Since the receiver and transmit antennas of these repeaters were usually disposed close together and usually had isotropic antenna patterns, switching mechanisms were developed for switching the local oscillator between the down-converter and the up-converter.
However, to achieve an intermediate frequency suitable for amplification, the fundamental frequency of the local oscillator signal could be no less than the difference of the radio frequency of a received signal, less the desired intermediate frequency. Therefore, the local oscillator signal was, itself, a radio frequency signal. To provide an appropriately amplified transmission signal, the power level of the repeater's local oscillator had to be substantially higher than the power of the transmitted signal. Unfortunately, such high power local oscillators, capable of operating at high microwave frequencies, are relatively inefficient and expensive in comparison to high power local oscillators operating at lower frequencies. For example, a local oscillator operating at 6 Gigahertz is less efficient and more expensive than a local oscillator operating at 3 Gigahertz. The inefficiency of prior art repeaters employing local oscillators operating at relatively high microwave frequencies affected the cost and useful life of the repeater by shortening the life of the repeater power source and requiring larger, more expensive, power sources.
Where it was desired to deploy such repeaters over hostile terrain which did not admit to convenient maintenance and removal, it was recognized that an expendable repeater, which would be abandoned at the termination of its useful life, would be highly suited for such applications. However, due to the requirements of the local oscillator, the cost and the complexity of the prior art repeaters made them too unreliable and expensive to be considered for such unattended, expendable use. There was, therefore, a need for an inexpensive, reliable repeater having a long operating life and which was suitable for expendable use.